A maraging steel has a very high tensile strength of around 2000 MPa and is therefore used for various applications such as an component requiring a high strength, for example, a rocket part, a centrifuge part, an aircraft part, a part for a continuously variable transmission of an automobile engine, or a die or the like.
The maraging steel usually includes an appropriate amount of molybdenum and titanium as a strengthening element, and can obtain a high strength by precipitating an intermetallic compound such as Ni3Mo, Ni3Ti or Fe2Mo through an aging treatment. A representative composition of the maraging steel including molybdenum and titanium is, by mass %, Fe-18%Ni-8%Co-5%Mo-0.45%Ti-0.1%Al.
Although the maraging steel can obtain a very high tensile strength, it includes a nonmetallic inclusion (hereinafter, referred to as merely “inclusion”) in the steel, such as a nitride or a carbonitride such as TiN or TiCN, or an oxide such as Al2O3 or Al2O3—MgO. Thus, fatigue failure may occur starting from a coarse inclusion in the steel.
Therefore, it has been proposed to improve a fatigue strength by making TiN or TiCN fine. The Applicant has proposed, for example in JP 2004-256909 A (Patent Literature 1) and WO 2005/035798 (Patent Literature 2), a method for making a nitride based inclusion such as TiN or TiCN fine by remelting a consumable electrode including magnesium by a vacuum arc remelting (hereinafter, referred to as “VAR”) process .